Polyarylene sulfides such as a polyphenylene sulfide are generally thermoplastic resins although some of them have a thermosetting property, and have excellent properties desired for engineering plastics such as high resistance to chemicals, good mechanical properties over a wide range of temperature and a high stiffness at high temperatures.
It is known that polyarylene sulfides can generally be produced by polymerizing a dihalogenated aromatic compound such as p-dichlorobenzene with an alkali metal sulfide such as sodium sulfide in a polar solvent (cf. Japanese Patent Publication No. 12240/1977).
However, conventional polyarylene sulfides have a low molecular weight and thus to obtain a high molecular weight product it has been necessary to cure the low molecular weight polyarylene sulfides by heat treatment, and that operation has been complex with the result being an expensive final product.
Heretofore, there has been known a method for producing a polyphenylene sulfide having a high molecular weight in which a lithium halide is used as a catalyst as described in, for example, U.S. Pat. No. 4,038,263.
However, this prior art method is disadvantageous since it cannot provide a polyphenylene sulfide having such a high molecular weight that no heat treatment of the resultant polymer is needed.
Further, there has been known methods for producing branched chain polyphenylene sulfides having high molecular weights in which a compound containing three or more halogen atoms are present in the reaction system as described in, for example, Japanese Patent Publication No. 87919/1979 and Japanese Patent Application (OPI) No. 197430/1984.
These methods, however, are also disadvantageous in that the resultant polymers tend to gel easily so that there occur problems on productivity and quality of the products. In addition, if the resultant polymers have a high flowability it is difficult to smoothly carry out molding of the polymers into various shaped products such as films, fibers, molded articles due to the fact that the polymers partly gel.
Another problem of the above-mentioned conventional methods for producing polyarylene sulfides is that the polyarylene sulfide produced contains a large amount of one or more salts such as sodium chloride which are by-produced during the polymerization reaction, and when the polyarylene sulfide is used as a material for parts of electric and electronic devices the insulation resistance to humidity of circuits is decreased, resulting in the occurrence of misoperation.